1. Field of the Invention
The present invention relates to an anti-skid control braking system (also called an anti-skid brake control system (ABS)) for a four-wheel drive vehicle which can determine whether the four-wheel drive vehicle is running in a two-wheel or four-wheel drive state without an installation of a special-purpose detecting device or sensor especially used for detecting whether the four-wheel drive is running in the two-wheel or four-wheel drive state, the anti-skid control braking system having a transfer mechanism which is so arranged and constructed as to be selectable between the two-wheel drive state in which a driving force generated by a revolution source (engine) is distributed only toward mainly driven road wheels and the four-wheel drive state in which the driving force is distributed toward both of the mainly driven road wheels and secondarily driven road wheels at a predetermined distribution ratio and a braking pressure controller which is so arranged and constructed as to independently control braking pressures of braking cylinders installed on road wheels to be braking controlled on the basis of road wheel speed detection values of road wheel speed sensors used for detecting respective road wheel speeds (velocities) of at least two front left and right road wheels (secondarily driven road wheels) and at least one of rear road wheels (mainly driven road wheels) to be braking controlled.
2. Description of the Related Art
A previously proposed four-wheel drive vehicle having the anti-skid control braking system is exemplified by a Japanese Patent Application First Publication No. Heisei 4-163264 (published on Jun. 8, 1992).
The above-identified Japanese Patent Application First Publication discloses the previously proposed anti-skid control method and system as follows:
A 4WD (four-wheel drive) selection switch selectably switches between a two-wheel drive (2WD) state and a four-wheel drive (4WD) state.
In the two-wheel (2WD) state, a driving force generated by an engine installed in the four-wheel drive vehicle is transmitted only to front left and right road wheels (or rear left and right road wheels). In the (rigid) four-wheel drive (4WD) state, the driving force generated by the engine is transmitted to the front left and right road wheels via a power transmission, a front differential, and a front road wheel side drive shaft and to the rear left and right road wheels via the power transmission, a center differential, a propeller shaft, a rear differential, and a rear road wheel side drive shaft.
When the 2WD state is selected via the 4WD selection switch (or 2WD state is detected), braking pressures acted upon the front left and right road wheels and both of rear road wheels are independently controlled upon a start of the anti-skid control. When the 4WD state is selected via the 4WD selection switch (4WD state is detected) and the anti-skid control is started, both of the front road wheel side drive train (system) and rear road wheel side drive train (system) are interconnected. At this time, when braking forces exerted on the front road wheels and the rear road wheels are independently controlled, a torsional torque on the drive train (front road wheel and rear road wheel side drive trains) is increased so that road wheel speeds of the front road wheels and rear road wheels tend to be oscillated. Therefor, for example, when a tendency of lock on the rear right road wheel occurs so that both of the braking pressures on the rear left and right road wheels are reduced simultaneously in accordance with, so-called, select-low principle (selecting lower road wheel speed), the braking pressure on the front right road wheel located at the same longitudinal direction as the rear right road wheel) is also reduced simultaneously by a predetermined pressure so that pressure reduction controls of the braking pressures between each of the front road wheels and a correspondingly located rear road wheels are substantially in the same phase with each other, thus oscillations on the respective road wheel speeds being effectively suppressed.
However, since the anti-skid control is executed in mutually different modes of operation between the 2WD state and the 4WD state in the four-wheel drive vehicle having the anti-skid control braking system described above, it is necessary to determine whether the four-wheel drive vehicle is running in the two-wheel drive state or in the four-wheel drive state during the execution of the anti-skid control.
To determine the drive state of the four-wheel drive vehicle, a switch signal derived from the 4WD selection switch may be utilized. However, in this case, the 4WD selection switch functions only to issue a command to switch the drive state between the 2WD state and 4WD state and the switch signal does not function to detect an actual drive state.
Therefore, in order to detect that the transfer actually switches from the 2WD state to the 4WD state or vice versa, it is necessary by means of a special-purpose detecting switch or sensor to detect an engaged state of a free running wheel or a switched state in a switching mechanism within the transfer.
Hence, the special-purpose switch which is capable of detecting such the drive state as described above is required and its drive state detection signal is accordingly required. Hence, in this case, a reliability of parts of the detecting switch (or sensor) does not only need to be secured but also need to modify a hardware such as an addition of a circuit element used to detect a failure in the parts of the detecting switch (or sensor) and to add a fail-safe logic after a detection of the failure thereof. Consequently, a system reliability is accordingly reduced due to the increase in numbers of the parts and a total cost of the anti-skid control braking system with the above-described detecting switch (or sensor) is increased.